Serine

(CC) Image: David E. Volk
Serine, one of the twenty common amino acids.

Serine, abbreviated as Ser or S, is one of the twenty common amino acids used by living organisms to build proteins. It is one of the smallest amino acids, being larger than only glycine and alanine. It is subject to phosphorylation by kinases because it is one of the three amino acids, together with threonine and tyrosine, that contain a hydroxyl group (OH) on the side chain. It can be thought of as $\beta$-hydroxylated alanine.